Modelling the persistent low-state $\gamma$-ray emission of the PKS 1510-089 blazar with electromagnetic cascades initiated in hadronuclear interactions

TL;DR

This paper investigates the persistent low-state gamma-ray emission of the blazar PKS 1510-089, proposing that hadronuclear interactions of protons in the broad line region produce electromagnetic cascades explaining observed gamma-ray excesses.

Contribution

It introduces a novel Monte Carlo calculation of electromagnetic cascades inside a blazar using photon fields derived from spectral synthesis, linking hadronic interactions to gamma-ray observations.

Findings

Gamma-ray excess above 20 GeV observed in PKS 1510-089

Electromagnetic cascades from hadronuclear interactions can explain this excess

First calculation of cascade spectrum using photon fields from spectral synthesis code

Abstract

Blazars may accelerate protons and/or nuclei as well as electrons. The hadronic component of accelerated particles in blazars may constitute the bulk of their high-energy budget; nevertheless, this component is elusive due to a high value of the energy threshold of proton interaction with photon fields inside the source. However, broad line regions (BLRs) of some flat spectrum radio quasars (FSRQs) may contain a sufficient amount of matter to render primary protons "visible" in $\gamma$ rays via hadronuclear interactions. In the present paper we study the persistent $\gamma$-ray emission of the FSRQ PKS 1510-089 in its low state utilizing the publicly-available Fermi-LAT data, as well as using the spectrum measured with the MAGIC imaging atmospheric Cherenkov telescopes. We find an indication for an excess of $\gamma$ rays at the energy range $\gtrsim 20$ GeV with respect to a simple baseline log-parabolic intrinsic spectral model. This excess could be explained in a scenario invoking hadronuclear interactions of primary protons on the BLR material with the subsequent development of electromagnetic cascades in photon fields. We present a Monte Carlo calculation of the spectrum of this cascade component, taking as input the BLR photon field spectrum calculated with the Cloudy code. To our knowledge, this is the first calculation of electromagnetic cascade spectrum inside a blazar based on a direct calculation of the photon field spectrum with a spectral synthesis code.